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Ohm's Law
'History' ----- No, Ohms’ law has nothing to do with calming humming or yoga whatsoever rather, Ohms’ law is a vital relationship in the understanding of circuitry. Ohms’ law was formulated by German scientist Georg Simon Ohm who discovered that the ratio of the potential difference (the voltage drop from one point to a second point) to the current is constant for any conductor. In other words, the resistance remains the same as the magnitude or the direction of the potential difference changes. Initially Georg Ohm used voltaic piles, but later used a thermocouple because the thermocouple was a more stable voltage source, finally, to measure current he used a galvanometer.He applied various voltage levels to his circuit and recorded the resulting current. Through his experimental data he observed that the voltage drop was directly proportional to the current. This means that if voltage is plotted against current, the data will resemble a straight line where the slope at any given point is the resistance of the conductor. Ohm's law first appeared in his book Die galvanische Kette, mathematisch bearbeitet in 1827 in which he gave his complete theory of electricity. His results were not readily accepted by other physicists because his law was based entirely on experimental data. Ohm was forced to resign from his teaching position and lived in poverty and shame. However, his work was eventually recognized by the Royal Society with its award of the Copley Medal in 1841. http://www.corrosion-doctors.org/Biographies/images/ohm.jpg This image was taken from http://www.corrosion-doctors.org/Biographies/images/ohm.jpg ----- R = V / I ----- = Definition of terms in the equation = R Resistance is the opposition that a material offers to the passage of an electric current. Resistance ( measured in ohm's ) determines how much current will flow through a component. A resistor is a device that uses elictricity to do work such as a light bulb, a hair straightener,or a micro-wave. Resistors can also be used to control voltage and current levels. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/examples/exLightbulb.jpg This image was taken fromhttp://resourcescommittee.house.gov/subcommittees/emr/usgsweb/examples/exLightbulb.jpg V Voltage is the difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit. Voltage comes from various sources that provide electric current; examples of these sources are batteries and electrical outlets. http://www.e-furniture.co.uk/assets/product_images/640909.jpg This image was taken from http://www.e-furniture.co.uk/assets/product_images/640909.jpg I Electric current, measured in amperes or amps, is a flow of charged particles through a circuit. Electric current is like the current of water flowing down a river in that both measure the quantity of matter traveling over a certain measurement of time. Current flows from points of high voltage to points of low voltage on the surface of a conductor. In circuits, current usually travels through wire. http://www.tekwire.com/multicore_012.gif this image was taken from http://www.tekwire.com/multicore_012.gif The resistance of a conductor, R, is measured in Ohm’s represented by the symbol omega Ω and is defined as the ratio of the potential difference, V, to the current, I. One ohm (1Ω) is the resistance that permits a current of 1 A to flow when a potential difference of 1 V is applied across the resistance. Resistance can be thought of as the energy needed to move matter through a pipe for example. A clogged pipe will have more resistance than a clean one will. R = V / I Note : If you are planning to take the SAT II in physics, this equation should be memorized and you should be fluent with all aspects of Ohm's law. You will definetly see this topic on the test. '' From this equation we can extract some useful facts: *I is indirectly proportional to R therefore more resistance results in less current flow . * V is linearly proportional to I therefore more voltage will results in more current flow. * R is independent of V therefore it is a constant and cannot be changed. In simpler terms, a steady increase in voltage in a circuit with constant resistance, results in a constant linear rise in current. In this graph of voltage vs. current as current increases voltage does also. This idea is intuitive; imagine your faucet is slightly clogged, by increasing the voltage or energy used to pump water through the pipes, water would come o ut of the faucet at a faster rate. You should also take note that the slope of the graph is the resistance of the conductor. At any given point the ratio V/I is equal to a constant resistance, R. ----- 'Circuits' The definition of the word circuit is " closed path". An electrical circuit is a configuration where electric charges travel in a closed path. An electric circuit can be as simple as a battery, which provides voltage, and a single resistor such as a light bulb connected by wires. In circuitry we use symbols to represent components of a circuit. Become familiar with these symbols you may be asked to draw a circuit using circuit symbols. ----- 'Non-Ohmic Conductors' Not all conductors obey ohms law. In fact, most conductors of electricity are non-ohmic conductors.Ohm's law is only true for resistors whose resistance does not depend on the applied voltage, which are called ohmic devices. Temperature is an important factor in determining the accuracy of Ohm's law. When the temperature of the metal increases, the collisions between electrons and atoms increase. When a substance heats up because of electricity flowing through it, the resistance will increase thereby defying Ohm's law which states that resistance is a constant. With non-ohmic conductors, resistance depends on voltage and is no longer a constant. To determine whether a material is ohmic or not, one can plot voltage versus current and observe whether the data follows a linear path. At very high currents, the material may overheat and melt, causing an open circuit. At very high voltages, the material may be damaged by electric breakdown or arcing, causing a term most of us have heard of but never really understood,a short circuit. Some materials have resistances that change with magnetic field intensity, light intensity or other variables. The resistance of a conductor depends on : *The nature of the material *The temperature *The shape of the conductor Keep in mind these non-ohmic conductors * Batteries * Semi-conductors * Light bulbs * Capacitors ----- '''Practice Problems' Regents level questions: 1) For any point on the line, what does the ratio of V to I represent? 2) When a conductor has a potential difference of 100 volts placed across it, the current through it is 5 ampere. What is the resistance of the conductor? 3) The current in a circuit is supplied by a generator. If the resistance in the circuit is increased, the force required to keep the generator turning at the same speed is (1) decreased (2) increased (3) unchanged 4) If the potential difference across a 50- ohm resistor is 5 volts, what is the current through the resistor? (1)10A (2) .5A (3) 5A (4) .1A 5) If the voltage across a 4-ohm resistor is 12 volts, the current through the resistor is (1) .25 A (2) .48 A (3) 3.0A (4) 4.0A 6) A resistor carries a current of .1 ampere when the potential difference across it is 5 volts. The resistance of the resistor is (1) .02 Ω (2) .5Ω (3) 5Ω (4) 50Ω 7) If the potential difference across a 12-ohm resistor is 6 volts, the current through the resistor is (1) .33 A (2) .5A (3) 3 A (4) 4 A Solutions to these problems ----- Resources and References =''Resources'' = http://jersey.uoregon.edu/vlab/Voltage/ This website offers an experiment that allows you to apply various resistors and voltage to a circuit. If you apply too much resistance to the circuit, the lightbulb will receive too much current and will break if you apply too little, the light will not turn on resistance. Test your knowledge of Ohm's law by trying to apply the right amount of voltage and resistance to get the lightbulb to light up! http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmlaw.html This website offers a clear explanation of Ohm’s law and also explains current law and voltage law. http://www.kpsec.freeuk.com/ohmslaw.htm Here you will find a concise explanation of Ohm’s law with helpful tips for answering questions. http://www.regentsprep.org/Regents/physics/phys03/bsimplcir/default.htm This site offers an explanation on the simple circuit and ohms law from New York state high school regents exam prep. This site is ideal for students studying for the physics regents. http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/ohmslaw.htm A lesson on Ohms law from New York state regents prep site. http://www.sparknotes.com/testprep/books/sat2/physics/chapter14section3.rhtml If your studying for the SAT II in Physics, this site covers all the topics using clear explanations. =''References'' = Glencoe Physics- Principles and Problems, by Paul Zitzewitz. Glencoe/McGraw-Hill, 1998. This texbook gives clear explanations with a variety of problems to practice from and also offers informative experiments that can often be done with household materials. Barron's Regents Review for Physics, by Miriam A. Lazar, third Edition. Barron's Educational Series, Inc. 2004. This review book is like a bible to students studying for the phsics regents. It clearly explains all the topics on the regents curriculum and gives realistic preactice problems with two actual New York State Regents exams.